Many elevator cars are equipped with sensors that provide load signals to the elevator controller indicative of the elevator car weight. The measured weight can be used by the controller to precisely pretorque the motor prior to releasing the brake, to implement dispatching strategies, such as bypassing hall calls when the car is full, or indicating an overload condition. For bypassing hall calls or indicating an overload condition, the load signal is compared with a predetermined load value stored in the elevator controller.
Many sensor systems are subject to drift over long periods of time, resulting in the need for calibration. In the context of elevator car load weighing, sensor drift can result in jerky starts when the motor is incorrectly pretorqued. This can also result in the improper implementation of dispatching strategies, such as hall call bypass when the car is not fully loaded.
The usual method of calibrating the load sensors is for service personnel to bring a calibration weight to the building, and measure the sensor signals under different load conditions. This method requires putting the massive calibration weight on a weight cart and usually wheeling it over the lobby carpet to the elevator car. This wears out the carpet as well as the mechanic assigned to recalibrate the load weighing system.
Not only are weight carts needed for calibration, worse, they are needed for recalibration. The need for recalibration might be observed when the car 2 experiences roll back or roll forward. A passenger in the car may observe roll back when he sees the car move just before the doors close. Roll back occurs when the car rolls in a direction opposite to the direction of travel to the next landing. Roll forward occurs when the car lurches in the same direction that the car takes when it moves to service the next landing. Both are caused by improper pretorquing of the motor. The motor is normally pretorqued so that when the brake is lifted, the car does not move. However, for the proper amount of armature current to be applied the car load must be known. If it is not, too much or too little pretorque is applied and roll back or roll forward occurs.
One way to recalibrate without bringing a calibration weight back to the elevator site is to leave it there when calibrating. This is described in U.S. Pat. No. 4,674,605, "Automatic Elevator Load Sensor Calibration System". There, an empty elevator car is caused to lift a predetermined calibration weight located above the normal car excursion, above the top floor. The car load sensor output before and after lifting the calibration weight, together with the known values for the empty car weight and the calibration weight, are used to recalibrate the load versus signal function. While this system avoids the need for hauling a calibration weight from outside the building into the building to calibrate the load weighing system, it requires a calibration weight for each shaft and room above the normal floor excursion to store that calibration weight.